Sierra Gorda is situated close to the eastern margin of the plain known as the "intermediate depression" (Llaumett, 1994) or the Intermediate Valley (Sillitoe and McKee, 1996), that separates the Cordillera de la Costa to the west and the Cordillera de los Andes to the east, close to where the plain gives way to the pre-Cordillera.

Sierra Gorda is located in the overall northerly trending Paleocene belt of porphyry copper deposits, which corresponds approximately to the Intermediate Valley. This belt is east of the Cretaceous (~130 Ma) western belt, and ~50 km west of the Oligocene deposits, and includes Guanuco, Lomas Bayas and Spence in Northern Chile, and Toquepala, Cuajone and Cerro Verde in southern Peru (Awmack, 2004b). Sierra Gorda lies in the centre of a local cluster of prospects representing a northeasterly, 1 to 3 km wide x 13 km long cross-trend.

There are two main successions in the Sierra Gorda district:i). a sequence of early Cretaceous, slightly deformed, andesitic to rhyolitic volcanic and associated sedimentary rocks (tuffs and volcanosedimentary sandstones and siltstones) which are found in a discontinuous NNE-trending, 30 to 50°E dipping, belt that extends over tens of kilometres through the Intermediate Valley, and ii). an early Tertiary (64 to 59 Ma) batholithic intrusive suite that includes both the main equigranular batholith and intruding porphyry phases. Monzodioritic to granodioritic to granitic rocks outcropping in the area belong to a Paleocene batholith extending over tens of kilometres, with local marginal phases and hypabyssal intrusive rocks of different composition, including some with microporphyritic to porphyritic textures. The hypabyssal intrusions locally cut the batholith and surrounding volcanic rocks and have associated hydrothermal alteration, breccia facies, and metallic mineralisation.

Four main fault systems are present in the Sierra Gorda mining district: i). north to NNE regional trend, possibly related to the majo Atacama Fault zone; ii). WNW, sub-vertical normal faults; iii). NE faults including fractures, veinlets and dykes; and iv). east-west structures that may have displaced some of the mineralised bodies.

Within the Sierra Gorda area, there are several centres of copper-molybdenum-gold-mineralisation, including the Salvadora and blind 281 sulphide mineral deposit, which together with the Catalina and Isabela (285 Zone) deposit comprise the Catabela mega-deposit with primary copper, molybdenum and gold mineralisation. Strong hydrothermal alteration is associated with the Tertiary intrusive dykes and apophyses within the Sierra Gorda district. The alteration zones occur as haloes surrounding breccia zones which are associated with intrusive rocks. These breccias an important structural features that often enclose high grade mineral concentrations, and include hydrothermal, intrusion and tourmaline breccias that appear to represent the most favourable zones for mineral deposition.

The Sierra Gorda deposits are hosted by Early Cretaceous rocks comprising of a sequence of andesites, mostly coarse- to fine-grained porphyritic andesites and andesite flows with intercalated agglomerates, tuffs, sandstones and siltstones, correlated to the 111±3 Ma Quebrada Seca Formation. These volcanosedimentary rocks are cut by Paleocene plutonic intrusive rocks of variable composition from monzodiorite, granodiorite to granite which are apparently related to the underlying Batholith, occurring as irregular stocks, dykes and small plugs. These rocks have variable textures, granularities and mineral compositions resulting in syenogranite in the Catalina, Salvadora and San Armando zones; monzogranite in the Catalina area; porphyritic quartz-bearing monzodiorite which occurs in the Catalina, Salvadora and San Armando areas; granite in the Salvadora area, and a feldspar porphyry throughout the Sierra Gorda district.

The Sierra Gorda mineralisation occurs as:i). Hypogene sulphides, which forms the bulk of the known mineralisation, both in terms of volume, as well as contained metal, and consist dominantly of chalcopyrite, although bornite is locally present as an accessory. Chalcopyrite mineralised rocks exist from below the leached zone to >1000 m below the surface. Hypogene molybdenite occurs in distinct bodies within breccia zones at Catalina and Salvadora, but elsewhere it is weak to absent. Gold typically accompanies the copper sulphides.ii). Leached/oxide zone, which is the product of in situ oxidation of the hypogene sulphides, extends from the surface to depths variably up to 200 m. It is grouped into copper-rich and copper-leached or barren zones. The most important copper oxide minerals are atacamite, brochantite, chrysocola and vermiculite. Hypogene molybdenite is typically oxidised to ferrimolybdite, powellite and grenite.iii). Supergene copper sulphides, occurring as an irregular zone of secondary copper sulphide enrichment, dominated by chalcocite over a variable thickness of 10 to 150 m, generally at the boundary between the oxidised and hypogene zones, although it can exist well down into the hypogene zone or some distance up into the oxide zone, depending on structural conditions. While a well-developed chalcocite enrichment blanket is absent, there is a significant quantity of supergene chalcocite present locally, generally at or near the current water table. Supergene processes took place somewhere between ~44 and 14 Ma.

The earliest hypogene veins generally comprise barren quartz, tourmaline-(±quartz), or magnetite-feldspar-(albite, ±quartz, ±K feldspar?) centre-lines, with or without visible halos of the same minerals. Slightly later veins, defining a potassium silicate event, commonly contain biotite and/or K feldspar as infill or halos. These K feldspar-bearing veins account for much of the molybdenite at Sierra Gorda. No significant chalcopyrite or pyrite appears have been deposited in these veins. K silicate alteration was also locally accompanied by graphic-textured, coarse-grained (up to 2 cm) pegmatitic quartz-K feldspar-albite-(molybdenite) 1 to 20 cm wide dykes and have conspicuous alteration halos of secondary biotite. Many of these dykes are <10 cm wide and appear to define a 150m wide north-trending zone in the 281 Zone.

Pervasive hydrothermal secondary biotite alteration is related to K-silicate veinlets, and as disseminated secondary biotite within large volumes of andesites and tuffs. K-silicate veins containing molybdenite are much more common in intrusive rocks (especially syenogranite, granodiorite porphyry, and the feldspar porphyries, but also in monzogranite and
monzodiorite) than in volcanic wall rocks. The volcanic rocks, in contrast appear to contain volumetrically more K-silicate alteration as secondary biotite and lesser secondary K feldspar than most of the intrusive rocks. Veins with chlorite-sericite-anhydrite-(±clay) in vein centres or as halos post-date K-silicate veins. Most 281 Zone copper mineralisation appears to be associated with these chlorite-sericite veins, which seem to represent a weak phyllic or intermediate argillic alteration stage, although it is unclear if the clay minerals in chlorite-sericite-altered rocks are hypogene or supergene in origin.

Disseminated chlorite is regionally distributed, particularly in the Cretaceous volcanic rocks, although closer to the mines, it typically is associated with epidote in veins and vein halos, forming a true propylitic assemblage. The epidote-bearing veins are typically associated with up to 1% pyrite in veins and disseminations as well as rare chalcopyrite. Propylitic alteration is generally cut by the chlorite-sericite suite of veins and may be contemporaneous with, or locally later than, K-silicate veins and alteration.

Texture-destructive quartz-sericite-pyrite alteration is generally very strongly pyritic and post-dates the K-silicate, chlorite-sericite (phyllic), and chlorite-epidote (propylitic) assemblages. Late base-metal veins with quartz-pyrite-chalcopyrite-(±sphalerite, galena, and/or arsenopyrite), with sericitic halos, cross cuts these quartz-sericite-pyrite veins. The latest fractures may contain hydrothermal carbonate (siderite or dolomite) or still later supergene gypsum.

Pulses of breccia intrusion invaded and partly broke up their host rocks, both early and late in the development of the Sierra Gorda batholith and its hypabyssal intrusions. Llaumett (1994) observed that the breccias are all hydrothermal in origin, the result of forceful liberation of hydrothermal fluids along favourable structures and permeable zones. At Sierra Gorda, there is a sequential series of breccias with differing structural and compositional characteristics. Clasts have highly variable sizes, typically from 1 to 10 cm across, ranging from monomictic to polymictic. The matrix cement are typically composed of mixtures of hydrothermal minerals including quartz, K feldspar, chlorite, tourmaline, sulphides, anhydrite and sideritic to dolomitic carbonate. Llaumett (1994) describes pre-, syn and post-mineralisation breccias. The early pre-mineralisation breccias have weakly altered clasts and did not generally produce sulphide mineralisation and include both (very early) intrusive breccias and (slightly later) tourmaline breccias.

Several breccia bodies exhibit a close temporal and spatial relationship with mineralisation, although petrographic studies indicate much of the mineralisation post-dates brecciation. Among the most important are the Brecha Catalina, Brecha Salvadora and Brecha Olvidada.

The Brecha Catalina is a large body that contains >1 cm clasts of granodiorite porphyry, syenogranite, monzodiorite or andesite in a cement dominated byK feldspar (45 to70 vol.%) with lesser to significant chlorite and quartz (&plusmnsericite), all of which typically replace tourmaline. Sulphides are typically associated with chlorite (±sericite) alteration and are largely confined to breccia clasts and local chlorite-sericite-sulphide veinlets that cross cut the fragments and matrix. It forms a near-vertical pipe with a surface dimensions of roughly 600 x 30 to 150 m.

The Brecha Olvidada occurs at the Salvadora deposit and contains moderately angular fragments of syenogranite and/or microgranite that are typically 1 to 3 cm in size but range up to 10 cm or more. Its matrix is an aggregate of rock fragments of the same composition but with crystals of tourmaline in a cement is a microcrystalline aggregate of sericite, quartz, potassium feldspars and acicular crystals of tourmaline. Sulphides occur in interstitial spaces in the matrix. It has near-vertical sides, with a half-moon shape in plan view, convex towards the SE, with an apparent thickness at the surface of ~50 m, with a tip to tip length of ~850 m.

The Brecha Salvadora also has a "half moon" shape convex to the SE, roughly 75 m NE of, and broadly concentric with the Brecha Olvidada. The Brecha Salvadora is smaller than the Brecha Catalina and exhibits less evidence of multiple stages than does the Brecha Catalina. The Salvadora breccia complex may represent a deeper level of exposure than the Catalina complex, as the Brecha Salvadora is now only found near the present erosional surface. Tourmaline-lined, sheeted fractures extend for a few metres outwards from the Brecha Salvadora but do not contain appreciable copper and molybdenum (Sillitoe,1993).

The late, post-mineralisation, breccias intrude the earlier phases, and typically contain clasts of varied rock types and are poorly mineralised. The two dominant types of late breccias are charcaterised by pyrite-(quartz-sericite) and carbonate assemblages, both of which appear to be volumetrically small, irregular and discontinuous.